This invention relates generally to apparatus for receiving and combining together a plurality of modulated signals and, more particularly, to apparatus of this kind that controllably weights the various signals being combined so as to null out an interference signal superimposed on each received modulated signal. Null processing receivers of this general type are useful in a variety of applications, but present invention is described in this specification in the context of a Global Positioning System (GPS) receiver for processing modulated radio-frequency (rf) signals received from multiple orbiting satellites, and accurately determining the position of the receiver.
In the presence of an interference signal, referred to as a jamming signal, received from an unpredictable and usually variable direction, a multi-element receiver antenna array may be used to minimize or null the effect of the interference. Such a system is described in the present inventor's prior patent, U.S. Pat. No. 4,734,701, entitled "Null Processing Receiver Apparatus and Method," to be referred to as the prior Grobert patent.
In a system of this general type, the antenna array elements receive modulated radio-frequency (rf) signals, which are typically summed together to produce a sum signal for subsequent down-converting, demodulation and baseband processing. Prior to summation, each elemental rf signal is controllably adjusted in amplitude and phase (an adjustment referred to as complex weighting), in such a manner as to null or cancel out the interference signal. This adaptive interference cancellation is usually performed in a way that maximizes the power of the sum signal, since it is assumed that the power of the interference signal greatly exceeds that of the desired information signal.
The prior Grobert patent discloses a system for avoiding the use of complicated and often unreliable hardware for complex weighting of the antenna signals. Basically, a hardware section of the system downconverts each rf signal received by an antenna element to an intermediate frequency, demodulates the signal to remove encoded data in the form of a pseudorandom (pn) code (included in the signals transmitted from satellites in the Global Positioning System), and mixes the signal with reference quadrature (I and Q) signals, to produce pairs of analog baseband data signals: I.sub.1, Q.sub.1 ; I.sub.2, Q.sub.2 ; I.sub.n, Q.sub.n. Then the hardware filters these analog signals and converts them to digital form for further processing in a software section of the system. The software section generates an I.sub.NULL signal and a Q.sub.NULL signal. The I.sub.NULL signal is obtained by summing a primary I signal, such as I.sub.1, with weighted versions of the auxiliary I signals, such as I.sub.2, I.sub.3, . . . I.sub.n. Each weighting circuit includes two multipliers and an integrator, and functions to multiply an auxiliary signal by a weighting factor generated by correlating the auxiliary signal with the summer output The Q.sub.NULL signal is generated in a similar fashion. The weighting circuits cooperate to cancel out the effect of a jamming signal, as explained in the prior Grobert patent, and the nulled I and Q signals are then used in tracking and detection circuitry.
The system described in the prior Grobert patent, and prior systems used for the same general purpose, produce a composite antenna pattern that presents a null region in the direction of a jamming signal source. Multiple jamming sources are handled in the same way, if the antenna array has enough elements, and the antenna pattern presents multiple nulls to the multiple jamming signal sources. Unfortunately, the resulting antenna pattern may also have spurious nulls that are directionally aligned with desired information signal sources, such as GPS satellites. Each spurious null reduces the signal energy from a signal source with which it may be aligned, and can also induce "cycle slips" in a carrier tracking phase lock loop in the receiver.
GPS receivers, as is generally well understood, include a number of synchronizing circuits, such as phase lock loops, to synchronize operation of the receiver with the incoming signals. Basically, a GPS signal consists of at least one rf carrier signal, modulated with data signals. One of the synchronizing circuits or tracking loops in the receiver is designed to lock onto the incoming carrier signal. If the incoming signal is weakened by a spurious antenna null, the carrier tracking loop may momentarily lose synchronism with the received carrier signal, and may thereby "slip" one or more cycles of the carrier. This is obviously disruptive to efficient operation of the receiver, and prevents continuous demodulation of satellite data. The present invention provides an elegant solution to this significant difficulty, and avoids associated carrier loop cycle slips.